1. Field of the Invention
The present invention relates to a method for producing powdered iron oxide used for manufacturing high-quality soft ferrite.
2. Description of the Related Art
Powdered iron oxides are widely produced by roasting and oxidizing a crude iron chloride solution. A hydrochloric solution after use for pickling of iron and steel, a solution obtained by dissolving iron and steel and or iron oxide in hydrochloric acid, the heated and concentrated solutions thereof or the solutions thereof whose insolubles are filtrated, are used for such a crude iron chloride solution.
Powdered iron oxide is used as a raw material for soft ferrite. FIG. 1 shows an example of a typical process for making powdered iron oxide. In FIG. 1, a waste hydrochloric pickling solution 6 is stored in a reservoir 1. The hydrochloric pickling solution 6 is contacted with hot gas 3 in a contact tower 2. Hot gas 3 containing hydrogen chloride is generated in a roasting furnace 5. By this contact the waste hydrochloric pickling solution is heated and concentrated in the contact tower 2. The heated and concentrated solution is then sprayed in the roasting furnace 5, and a sprayed mist of the solution is roasted and oxidized, thereby producing a powdered iron oxide 4 and hot gas 3. An electrostatic dust collector 7 collects the powdered iron oxide 4. The hydrochloric acid is recovered from hot gas 3 in a recovering tower 8. The numeral 9 represents spray water and reference number 10 is hot air. Iron and steel or iron oxide contains Si, Al, Cr, Cu, P and etc., and industrial water contains Ca. These elements are contained in the crude iron chloride solutions. Powdered iron oxide produced by roasting the crude iron chloride solution also contains those elements as impurities. This process of producing powdered iron oxide is very simple and the cost of the powdered iron oxide produced thereby is low. Accordingly, the powdered iron oxides made by this process are used widely as raw materials for soft ferrite of ordinary grades. However for high-quality soft ferrite the powdered iron oxides made by this process are not used because of the impurities contained therein.
Powdered iron oxide having a decreased content of SiO.sub.2 and CaO also may be produced by this process if certain additional treatment to the crude iron chloride solution is performed. However, the powdered iron oxide of decreased SiO.sub.2 and CaO content is still unsatisfactory for the high-quality soft ferrite, and this powdered iron oxide is not used for high-quality soft ferrite. For high-quality soft ferrite a high-purity powdered iron oxide of very low impurity content exclusively is used.
A high-quality powdered iron oxide is usually produced by a crystal refining method. In the crystal refining method crystals of iron sulfate or iron chloride are crystallized from an aqueous solution thereof, and the high-purity powdered iron oxides are produced by oxidizing these crystals. In this method, however, the impurities are not reduced sufficiently by a single occurrence of crystallization because some of the impurities remain included in the crystals. The thus obtained crystals are, therefore, dissolved again in water etc., and crystallized again. These treatments are repeated several times, thereby reducing the impurity content of the crystals. According to the crystal refining method a high-purity powdered iron oxide is obtained, in which the content of all impurities is very low. However, the crystal refining process is complicated and the production cost or the powdered high-quality iron oxide produced thereby is very high. Among the impurities in powdered iron oxide, some impurities may impair the properties of a soft ferrite, and others do not. However, general knowledge regarding the type of impurities which impair the ferrite properties has been insufficient, and it has been difficult to improve the powdered iron oxide of ordinary grades to a level sufficient for the high-quality soft ferrite.
Accordingly, it is an object of the invention to selectively remove harmful impurities from an iron oxide.
Another object of the invention is to simplify the process for producing iron oxide for high quality soft ferrite.
It is a further object of the invention to reduce the cost of producing iron oxide for high quality soft ferrite.